1. Field of the Invention
The present invention relates to a method and apparatus for rolling a strip. More particularly, the present invention relates to a method and apparatus for rolling a strip, which method or apparatus enables reliable rolling of uniform quality for producing thin strips, including foils having good shape.
As used herein, the term "shape" refers to surface shape and the term "good shape" refers to the surface of a strip in which uneven stretching has been suppressed, such as so-called center buckling in which stretching of a central portion of a strip in the width direction is greater than that of the edges; a so-called wavy edge in which stretching of the strip edges is greater than that of a central portion; and so-called quarter buckling in which stretching of outer quarter portions in the width direction of the strip is greater than that of the center and edges.
2. Related Background Art
Typically, a 4-hi rolling mill which has left and right pressing-down balancers, roll benders, and roll coolants is widely used for rolling strips of various metals. When such a rolling mill is employed, the shape of the strip is regulated by approximating the strip shape in the width direction by use of a power function and matching the strip shape to a target shape based on the approximation by means of the pressing-down balancers disposed on both the left and the right sides, the roll benders, and the roll coolants.
Japanese Patent Publication (kokoku) No. 20171/1993 discloses a method for regulating the strip shape in which a variable crown roll having a single oil chamber serves as a back-up roll and a strip shape in the width direction detected by a shape detector is approximated by a function including terms of the first, second, and fourth (or sixth) powers of a distance measured from the center of the width, each term of the power function being controlled to match a corresponding target value. Specifically, the term of the first power is controlled by adjusting the amount of the left and right pressing down (hereinafter called "pressing-down amount"); the term of the second power by adjusting the roll crown of the variable crown roll having a single oil chamber; the term of the fourth power or the term of the sixth power by adjusting the bending force of a roll bender.
However, when a rolling apparatus employing a small-diameter work roll having a ratio of barrel length L to diameter D (L/D) is more than 4, such as a rolling apparatus employed for rolling thin strips such as aluminum foil, controlling the term of the fourth power is substantially difficult, because portions effectively controlled by a roll bender are limited to the ends of the roll.
In addition, when a thin strip having an exit-side-thickness as low as about 30 .mu.m or less is rolled, ends of the upper and lower work rolls come into contact with each other at their end portions, due to the small thickness of strip. In such a situation, a work roll bender which controls the strip shape by utilizing the bending force of a work roll does not function effectively.
In this case, control by roll coolants is more important. However, roll coolants disadvantageously require a warm-up process prior to rolling, and in addition, the performance thereof is unsatisfactory and response during operation is poor.
Therefore, the conventional controlling schemes involve problems that sufficient control precision cannot be attained, particularly in the rolling of thin strips such as foil.